rev-list: support human-readable output for `--disk-usage`

rev-list: support human-readable output for `--disk-usage`

The '--disk-usage' option for git-rev-list was introduced in 16950f8384
(rev-list: add --disk-usage option for calculating disk usage, 2021-02-09).
This is very useful for people inspect their git repo's objects usage
infomation, but the resulting number is quit hard for a human to read.

Teach git rev-list to output a human readable result when using
'--disk-usage'.

Signed-off-by: Li Linchao <lilinchao@oschina.cn>
Signed-off-by: Junio C Hamano <gitster@pobox.com>